SMS4 Encryption Algorithm for Wireless Networks

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Tyrone Berry
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1 SMS4 Encryption Algorithm for Wireless Networks Translated and typeset by Whitfield Diffie of Sun Microsystems and George Ledin of Sonoma State University 15 May 2008 Version 1.03 SMS4 is a Chinese block cipher standard, mandated for use in protecting wireless networks, and issued in January The input, output, and key of SMS4 are each 128 bits. The algorithm has 32 rounds, each of which modifies one of the four 32-bit words that make up the block by xoring it with a keyed function of the other three words. Encryption and decryption have the same structure except that the round key schedule for decryption is the reverse of the round key schedule for encryption.

3 SMS4 Encryption Algorithm for Wireless Networks The SMS4 algorithm is a block cipher with 128-bit key and 128-bit input block. Encryption and decryption take 32 rounds of nonlinear substitutions. Encryption and decryption have the same structure, but the round key schedule for decryption is the reverse (goes in the opposite order) of the round key schedule for encryption. 1. Terminology (Definitions) 1.1 Zi and ZiJie (Word and Byte) Z2 e is the set of e-bit vectors. Specifically, the elements of Z2 32 are called Zi (32-bit words), and the elements of Z2 8 are called ZiJie (8-bit characters, or bytes). 1.2 S box The S (substitution) box takes in 8 bits and outputs 8 bits. It is written Sbox(.). 1.3 Fundamental Operations The two fundamental operations used by this algorithm are: the bitwise XOR of two 32-bit vectors, <<< i the circular shift of a 32-bit word, with i bits shifted left. 1.4 Input and output blocks, and key The 128-bit input block consists of four 32-bit words MK = (MK 1, MK 2, MK 3, MK 4 ) or MK i (i = 0, 1, 2, 3). The round key schedule, derived from the encryption key, is represented by (rk 0, rk 1,..., rk 31 ), where each rk i (i = 0,..., 31) is 32 bits long. The 128-bit output block consists of four 32-bit words F K = (F K 0, F K 1, F K 2, F K 3 ). For decryption, the round key schedule is represented by CK = (CK 0, CK 1,..., CK 31 ) or F K i (i = 0,..., 3), CK i (i = 0,..., 31). 2. The round function F This algorithm uses a nonlinear substitution structure, encrypting 32 bits at a time. This is called a one-round exchange. To illustrate, consider a one-round-substitution: Let the 128-bit input block be the four 32-bit elements (X 0, X 1, X 2, X 3 ) (Z2 32 ) 4, with rk Z2 32, then F is given by F (X 0, X 1, X 2, X 3, rk) = X 0 T (X 1 X 2 X 3 rk) 2

4 2.1 Mixer-substitution T T is a substitution that generates 32 bits from 32 bits T : Z2 32 Z2 32. This substitution is a reversible process. It consists of a non-linear substitution, τ, and a linear substitution L, i.e., T (.) = L(τ(.)) Non-linear substitution τ τ applies 4 S-boxes in parallel. Let a 32-bit input word be A = (a 0, a 1, a 2, a 3 ) (Z 8 2) 4, where each a i is an 8-bit character. Let the 32-bit output word be B = (b 0, b 1, b 2, b 3 ) (Z 8 2) 4, given by (b 0, b 1, b 2, b 3 ) = τ(a) = (Sbox(a 0 ), Sbox(a 1 ), Sbox(a 2 ), Sbox(a 3 )) Linear substitution L B Z2 32, the 32-bit output word of the non-linear substitution τ will be the input word of the linear substitution L. Let C Z2 32 be the 32-bit output word generated by L. Then C = L(B) = B (B <<< 2) (B <<< 10) (B <<< 18) (B <<< 24) 2.2 S box All Sbox numbers are in hexadecimal notation a b c d e f 0 d6 90 e9 fe cc e1 3d b7 16 b6 14 c2 28 fb 2c b 67 9a 76 2a be 04 c3 aa c f4 91 ef 98 7a b 43 ed cf ac 62 3 e4 b3 1c a9 c9 08 e df 94 fa 75 8f 3f a a7 fc f ba c 19 e6 85 4f a b 81 b da 8b f8 eb 0f 4b d e 24 0e 5e d1 a c 3b d f d c e7 a0 c4 c8 9e 8 ea bf 8a d2 40 c7 38 b5 a3 f7 f2 ce f a1 9 e0 ae 5d a4 9b 34 1a 55 ad f5 8c b1 e3 a 1d f6 e2 2e ca 60 c ab 0d 53 4e 6f b d5 db de fd 8e 2f 03 ff 6a 72 6d 6c 5b 51 c 8d 1b af 92 bb dd bc 7f 11 d9 5c 41 1f 10 5a d8 d 0a c a5 cd 7b bd 2d 74 d0 12 b8 e5 b4 b0 e a 0c e 65 b9 f1 09 c5 6e c6 84 f 18 f0 7d ec 3a dc 4d ee 5f 3e d7 cb For example, if the input to the Sbox is ef, then go to e-th row and f-th column, to find Sbox( ef )= 84. 3

5 3. Encryption and decryption Let the reverse substitution R be: R(A 0, A 1, A 2, A 3 ) = (A 3, A 2, A 1, A 0 ), A i Z 32 2, i = 0, 1, 2, 3. Let the plaintext input be (X 0, X 1, X 2, X 3 ) (Z2 32 ) 4, the ciphertext output be (Y 0, Y 1, Y 2, Y 3 ) (Z2 32 ) 4, and the encrypting key be rk i Z2 32, i = 0, 1, 2,..., 31. Then encryption proceeds as follows: X i+4 = F (X i, X i+1, X i+2, X i+3, rk i ) = X i T (X i+1 X i+2 X i+3 rk i ), i = 0, 1,..., 31 (Y 0, Y 1, Y 2, Y 3 ) = R(X 32, X 33, X 34, X 35 ) = (X 35, X 34, X 33, X 32 ). This algorithm s encryption and decryption methods have the same structure, except the order in which the round keys are used is reversed. The key order for encryption is: (rk 0, rk 1,... rk 31 ). The key order for decryption is: (rk 31, rk 30,... rk 0 ). 4. Key expansion when encrypting The rk i round key used for encrypting in this algorithm is derived from the encryption key MK. Let MK = (MK 0, MK 1, MK 2, MK 3 ), MK i Z2 32, i = 0, 1, 2, 3; K i Z2 32, i = 0, 1,..., 31; rk i Z2 32, i = 0, 1,..., 31; the derivation proceeds as follows: First, (K 0, K 1, K 2, K 3 ) = (MK 0 F K 0, MK 1 F K 1, MK 2 F K 2, MK 3 F K 3 ) Then for i = 0, 1, 2,..., 31: rk i = K i+4 = K i T (K i+1 K i+2 K i+3 CK i ) Notes: (1) T substitution uses the same T as in encryption, except the linear substitution L is changed to L : L (B) = B (B <<< 13) (B <<< 23); (2) The system parameter F K, given in hexadecimal notation is F K 0 = (a3b1bac6), F K 1 = (56aa3350), F K 2 = (677d9197), F K 3 = (b27022dc). (3) The constant parameter CK is calculated as follows: Let ck i,j be the j-th byte of CK i,j (i = 0, 1,..., 31; j = 0, 1, 2, 3), i.e., CK i = (ck i,0, ck i,1, ck i,2, ck i,3 ) (Z 8 2) 4, then ck i,j = (4i + j) 7 (mod 256). The 32 constants CK i are represented in hexadecimal as tabulated below e15, 1c232a31, 383f464d, 545b6269, 70777e85, 8c939aa1, a8afb6bd, c4cbd2d9, e0e7eef5, fc030a11, 181f262d, 343b4249, 50575e65, 6c737a81, 888f969d, a4abb2b9, c0c7ced5, dce3eaf1, f8ff060d, 141b2229, 30373e45, 4c535a61, 686f767d, 848b9299, a0a7aeb5, bcc3cad1, d8dfe6ed, f4fb0209, 10171e25, 2c333a41, 484f565d, 646b7279 4

6 5. Encryption examples Below are encryption examples of this algorithm s ECB (electronic code book mode) calculation method. We use this to verify the correctness of this algorithm s encryption. The numbers are represented in hexadecimal notation. Example 1: Encrypt plaintext with key once plaintext: ab cd ef fe dc ba encrypting key: ab cd ef fe dc ba rk and the output in each round: rk[ 0] = f12186f9 X[ 0] = 27fad345 rk[ 1] = 41662b61 X[ 1] = a18b4cb2 rk[ 2] = 5a6ab19a X[ 2] = 11c1e22a rk[ 3] = 7ba92077 X[ 3] = cc13e2ee rk[ 4] = f4 X[ 4] = f87c5bd5 rk[ 5] = 776a0c61 X[ 5] = rk[ 6] = b6bb89b3 X[ 6] = 77f4c297 rk[ 7] = X[ 7] = 7a96f2eb rk[ 8] = a520307c X[ 8] = 27dac07f rk[ 9] = b7584dbd X[ 9] = 42dd0f19 rk[10] = c30753ed X[10] = b8a5da02 rk[11] = 7ee55b57 X[11] = fa rk[12] = c X[12] = 8b952b83 rk[13] = 30d895b7 X[13] = d42b7c59 rk[14] = 44ba14af X[14] = 2ffc5831 rk[15] = a1 X[15] = f69e6888 rk[16] = d120b428 X[16] = af2432c4 rk[17] = 73b55fa3 X[17] = ed1ec85e rk[18] = cc X[18] = 55a3ba22 rk[19] = X[19] = 124b18aa rk[20] = e89e641f X[20] = 6ae7725f rk[21] = 98ca015a X[21] = f4cba1f9 rk[22] = c X[22] = 1dcdfa10 rk[23] = 99e1fd2e X[23] = 2ff60603 rk[24] = b79bd80c X[24] = eff24fdc rk[25] = 1d2115b0 X[25] = 6fe46b75 rk[26] = 0e228aeb X[26] = ad rk[27] = f1780c81 X[27] = 7b938f4c rk[28] = 428d3654 X[28] = 536e4246 rk[29] = X[29] = 86b3e94f rk[30] = 01cf72e5 X[30] = d206965e rk[31] = 9124a012 X[31] = 681edf34 ciphertext: 68 1e df 34 d e 86 b3 e9 4f 53 6e Example 2: Use the same encryption key and encrypt the plaintext again and again 1,000,000 times. plaintext: ab cd ef fe dc ba encrypting key: ab cd ef fe dc ba ciphertext: c7 c6 fd 27 1f f8 04 c3 3d 3f 66 5

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